The present invention generally relates to a method for producing an aluminum support for a printing plate, and particularly relates to a method for electrochemically surface-roughening and electrochemically denaturing an aluminum plate (including an aluminum alloy plate). Specifically, the present invention relates to a method for producing an aluminum support for a printing plate constituted by a uniformly surface-roughened aluminum plate suitable for an offset printing plate.
Conventionally, an aluminum plate has been used as a support for an offset printing plate. Usually, the surface of the aluminum plate is surface-roughened for the purposes of improving adhesion between the aluminum plate and a photosensitive layer provided thereon, holding damping water to be used in printing, and the like.
As the method for such surface roughening, a mechanical surface-roughening method, such as ball graining or brush graining, has been used. Alternatively, an electrolytic surface-roughening method for electrochemically surface-roughening the surface of an aluminum plate in an acid electrolyte such as hydrochloric acid or nitric acid has been used.
After the mechanical surface-roughening treatment such as ball graining, brush graining, or the like, an etching treatment in an alkaline solution has been generally performed so as to remove an abrasive used in the mechanical surface-roughening and make the surface shape well. After the electrochemical surface-roughening treatment, on the other hand, etching treatment in an alkaline solution has been ordinarily performed so as to remove a smut component mainly containing a generated aluminum hydroxide and to shape the surface. As the alkaline solution, generally sodium hydroxide has been used.
FIG. 6 shows an example of the conventional process in in which, after surface roughening, alkali etching and anodizing are performed so as to shape the surface-roughened support and to form an oxide coating. That is, first, a surface-roughened aluminum plate 1 is alkali-etched through ejection of an alkaline solution from spray nozzles 6 in the alkali etching step E. The remaining alkaline solution is removed through injection of clean water from spray nozzles 5 in the washing step W, and then the surface of the aluminum plate is neutralized through injection of a dilute acid aqueous solution from spray nozzles 7 in the neutralization step N. Thereafter, the aluminum plate is placed opposite to an anode of a DC power source 2 in an acid electrolyte 9 so as to be surface-electrolyzed in the cathodic electrolytic step, and then placed opposite to a cathode of the DC power source 2 so as to be surface-oxidized so that an oxide coating is formed in the anodizing step A.
As noted above, an electrolytic surface-roughening method for electrochemically surface-roughening the surface of an aluminum plate in an acid electrolyte has attracted attention. According to this electrolytic surface-roughening method, an aluminum plate having a uniformly roughened surface in which the mean roughness distribution is small in comparison with a plate produced using the conventional mechanical surface-roughening method is obtained. The conditions for obtaining such a roughened surface, however, are exceedingly narrow. If conditions such as the electrolyte composition, temperature, and the electrolytic condition are fixed, it is possible to easily obtain aluminum plates having extremely reduced scattering of products and having uniform performance. Since processing oil, atmospheric oxygen, and moisture are complicatedly intertwined with aluminum in rolling and processing, oxides or smut unevenly exist on the surface of an aluminum plate. If an aluminum plate in such a state is immediately electrolytically surface-roughened, a uniform surface-roughening treatment cannot be performed, thereby sometimes the roughening treatment of the plate is uneven. Such unevenness is undesirable. When a coating is formed on the aluminum surface, such unevenness causes deterioration in the adhesive property of the coating.
Therefore, conventionally, an aluminum material has been immersed in an acid aqueous solution or in an alkaline solution before an electrolytic surface-roughening treatment is performed to thereby remove processing oil, oxide, or smut and to dissolve a processed degenerated layer so that the surface is washed and made uniform.
A method in which removal of rolling oil or a natural oxide coating existing on the surface of an aluminum plate is performed in an alkaline solution such as sodium hydroxide prior to electrolytic surface roughening is known from Japanese Unexamined Patent Publication No. Sho-54-65607.
As the electrolytic surface-roughening method, there are generally known methods as disclosed, for example, in the U.S. Pat. Nos. 4,548,683 and 4,087,341. When electrochemical surface-roughening is performed using an AC current, carbon electrodes are usually employed as counter electrodes for the aluminum plate to be surface-roughened. When using carbon as counter electrodes, however, the carbon electrodes are dissolved because of deterioration of a binder, as described in Japanese Patent Publication No. Sho-61-48596. Then, according to the published patent, auxiliary electrodes are used and a current flowing in the main electrodes is shunted using rectifier devices such as diodes so that the quantity of the current flowing out from the main electrodes is reduced so as to be smaller than the current flowing into the main electrodes to thereby prevent the main electrodes from being dissolved. Examples of the application of this method are disclosed, for example, in of U.S. Pat. Nos. 4,533,444, 4,597,853 and 4,536,264.
As the method for electrochemically surface-roughening an aluminum plate in a neutral salt aqueous solution, on the other hand, a method disclosed in Japanese Unexamined Patent Publication No. Sho-52-26904 is known. Further, Japanese Unexamined Patent Publication No. Sho-59-11295 discloses a method for electrochemically denaturing the surface of an aluminum plate by cathodic electrolysis in a neutral salt aqueous solution. It is described that in a neutral salt aqueous solution of pH 6-8, which is a particularly advantageous condition, dissolved aluminum ions can be continuously removed from the neutral salt aqueous solution by filtration or centrifugal separation because the aluminum ions are precipitated in the form of aluminum hydroxide or aluminum oxide hydrate.
However, when a support for a printing plate is electrolytically surface-roughened, a washing treatment using sodium hydroxide is usually used for performing degreasing and removing a natural oxide coating before the electrolytic surface-roughening treatment, and a light etching treatment using sodium hydroxide is used to remove aluminum hydroxide generated in the electrolytic surface-roughening treatment and to shape the edge portions of formed pits after the electrolytic surface-roughening treatment. Both treatments involve a chemical dissolution reaction due to sodium hydroxide, and it has been therefore difficult to suppress the quantity of dissolution. Further, since a permeable membrane is used for removal of aluminum dissolved in the sodium hydroxide, the required liquid waste treatment cost is costly.
Further, since an auxiliary electrode used in the known method is provided for preventing dissolution of the main electrodes, and any reaction at the auxiliary electrode does not contribute to the surface-roughening reaction, equipment costs are high. For example, in the case of separately providing an auxiliary electrode cell as disclosed in U.S. Pat. No. 4,533,444, particularly, there has been a large disadvantage in equipment cost.
Therefore, a first object of the present invention is to eliminate the foregoing disadvantages in the prior art and provide a method for producing a support for a printing plate in which the conventional sodium hydroxide pretreatment such as for degreasing of an aluminum plate and smut removal is changed into an electrolytic treatment to thereby simplify liquid waste disposal. A part of an electrolytic surface-roughening reaction is performed by an auxiliary electrode, which has not directly contributed to the reaction in the conventional method, so that the process is further simplified to thereby improve production costs.
Further, in the immersion of an aluminum plate in an alkaline solution, for example, immersion in a sodium hydroxide aqueous solution, a large quantity of aluminum is dissolved, which reduces the life of the liquid. Also, because the immersion is a rapid chemical dissolving reaction, perforation or blowout in a sheet is liable to occur if the sheet is thin. Moreover, removal of metal ions, mainly, aluminum ions, from a sodium hydroxide aqueous solution has to depend on a method employing an ion-permeable membrane, which is relatively high in equipment cost.
In order to solve these problems, a method in which the surface of an aluminum plate is electrochemically denatured by cathodic electrolysis in a neutral salt aqueous solution has been disclosed in Japanese Unexamined Patent Publication No. Sho-59-11295, as noted above. To realize such a method, referring to FIG. 7, a surface-roughened aluminum plate 1 is subject to cathodic electrolytic treatment in the cathodic electrolytic step C in a neutral salt aqueous solution 8 at between an anode 3 and the aluminum plate 1, which is energized from a DC power source 2 through a conductor roll 10, so that the roughened surface of the support is shaped. The thus treated aluminum plate is washed with clean water in a washing step W, and then treated in an anodizing step A, as shown in FIG. 7. In this step, an oxide coating is formed on the aluminum plate surface in the same manner as in the case of FIG. 6.
In this method, however, there has been a disadvantage in that it is necessary to separately provide a power source to be used for cathodic electrolysis, and sparks are generated between the conductor roll and the aluminum plate because current conduction is performed using the conductor roll. As a result, holes are apt to be formed through the aluminum plate.
Therefore, a second object of the present invention is to overcome the limitations of the prior art in which a separate power supply must be provided for cathodic electrolysis, causing sparks to be generated which causes holes in the aluminum plate.
FIG. 11 shows another example of a conventional process. In this example, an aluminum plate 1 which has been subject to alkali-pretreatment and washed with water is electrolytically surface-roughened in an acid electrolyte 100 between the aluminum plate and main electrodes 4 using an AC power source 20. Next, the thus treated aluminum plate 1 is subject to cathodic electrolysis in a neutral salt aqueous solution between the aluminum plate and an anode 7 which is energized by a DC power source 2 through a conductor roll 10 to thereby remove aluminum hydroxide from the roughed surface. In this case, however, a possibility exists that holes will form in the aluminum plate by sparks generated between the conductor roll 8 and the aluminum plate.
Finally, although a washing treatment using sodium hydroxide is usually performed for the purposes of performing degreasing and removing a natural oxide coating before the electrolytic surface-roughening treatment, the treatment is a chemical dissolution reaction involving sodium hydroxide. Therefore, it has been difficult to suppress the quantity of dissolution. Further, there has been a disadvantage in that, since an etching treatment is performed using a strong alkali, holes are apt to be formed in the aluminum plate. Moreover, the liquid waste disposal cost is increased because a permeable membrane or the like must be used for removal of aluminum dissolved in sodium hydroxide.
Therefore, a third object of the present invention is to provide a method for producing a support for a printing plate in which holes are not formed in the aluminum plate due to a strong alkali used in the etching treatment.